This U.S. patent application is related to the following concurrently filed U.S. patent applications:
i) FREQUENCY PLAN FOR GPS RECEIVER by Najarian;
ii) INTEGRATED GPS RECEIVER ARCHITECTURE by Najarian et al.; and
iii) ALIAS SAMPLING FOR IF-TO-BASEBAND CONVERSION IN A GPS RECEIVER by Najarian et al., wherein these related U.S. patent applications are incorporated herein by reference in their entireties.
The present invention relates to GPS receivers, and particularly to a radio frequency system for use in GPS receivers that provides for GPS signal extraction in the presence of a much stronger interference signal.
The global positioning system (GPS) is based on an earth-orbiting constellation of twenty-four satellite vehicles each broadcasting its precise location and ranging information. From any location on or near the earth, a GPS receiver with an unobstructed view of the sky should be able to track at least four satellite vehicles thereby being able to calculate the receiver""s precise latitude, longitude, and elevation. Each satellite vehicle constantly transmits two signals, generally referred to as L1 and L2. The L1 signal from a satellite vehicle contains a unique pseudo-random noise code ranging signal with a chipping frequency of 1.023 MHz, system data with a bitrate frequency of 50 Hz, and an encrypted precise-code (y-code) with a chipping frequency of 10.23 MHz all being modulated onto a carrier frequency of 1575.42 MHz. The L2 signal consists of the system data and y-code being modulated onto a carrier frequency of 1227.60 MHz. In order to calculate a three-dimensional location, a receiver must determine the distance from itself to at least four satellite vehicles. This is accomplished by first determining the location of at least four satellite vehicles using ephemeris data received from the satellites. Once the locations of the satellites have been determined, the distance from the receiver to each of the satellites is calculated based upon an estimate of the receiver""s position. The measurement of the distance from the receiver to a satellite is based on the amount of time that elapsed between the transmission of a ranging signal from each satellite vehicle and the reception of that chip symbol by the receiver. In particular, the estimated position of the receiver is then corrected based upon a time epoch associated with the received ranging signal.
The accuracy of a GPS receiver depends on the accuracy with which the receiver is capable of measuring the time that has elapsed between the broadcast of the range information by a satellite vehicle and the reception of the information by the receiver. There are several factors that reduce the accuracy of the time measurement in the receiver design, including the sampling bandwidth of the receiver, the number of sampling bits, errors caused by internally generated noise, and external interference. Additional system factors that cause reduction of accuracy include errors in the ephemeris data (location of the satellite), errors caused by delays due to the ionosphere and troposphere, and multipath errors caused by reflected signals entering the receiver antenna.
Additionally, GPS receivers must be able to extract a very weak GPS signal, typicallyxe2x88x92133 dBm, in the presence of a much stronger jamming interference signal such as a television, radio, or microwave signal. Accordingly, there is a need for radio frequency circuitry capable of receiving the weak GPS signal while rejecting the interference signal.
The present invention provides a GPS receiver using a unique combination of fixed gain and variable gain amplifiers and signal quantization to achieve an integrated receiver that provides for GPS signal extraction with accurate position determination in the presence of interfering signal levels of up to xe2x88x9260 dBm. In general, the GPS receiver has downconversion circuitry that uses a first synthesizer output signal to reduce an amplified GPS signal to an intermediate frequency signal. Noise automatic gain control circuitry controls a first variable amplifier to provide an amplified intermediate frequency signal to filtering circuitry, wherein the intermediate frequency signal is amplified such that the receiver continues to operate linearly and GPS information carried in the amplified GPS signal is not compressed. A gain control circuit controls a second variable amplifier to amplify a filtered intermediate frequency signal from the filtering circuitry to a level sufficient to be digitized by sampling circuitry before baseband processing. The digitization circuitry provides for interference adaptation to maximize the signal-to-noise ratio of the intermediate frequency signal samples.
In one embodiment, the GPS receiver further comprises a low noise amplifier, which is capable of linear operation while receiving a GPS signal in the presence of up to xe2x88x9260 dBm interference noise, and sampling circuitry adapted to digitize the filtered intermediate frequency signal before baseband processing. In another embodiment, the low noise amplifier, downconversion circuitry, the first and second variable amplifiers, and the sampling circuitry are integrated into a single semiconductor die preferably fabricated in a SiGe semiconductor process. Additionally, the downconversion circuitry, the first and second variable amplifiers, and the sampling circuitry may have differential inputs and outputs. The use of differential signals allows common mode rejection thereby suppressing the effects of received noise.